Nonionizing Electromagnetic Fields and Cancer: A Review

Nonionizing Electromagnetic Fields and Cancer: A Review

We strongly agree with the authors that, although there is no
compelling evidence to suggest that nonionizing electromagnetic
fields represent a public health hazard, there is sufficient evidence
of magnetic- and electric field-induced biologic effects to continue
scientific investigation of this issue.

Just as it would be inadequate to investigate the action of a
few chemicals and decide that there is no harm possible from any
chemical in the environment, a more informed strategy is needed
in studies of electric and magnetic fields. Salvatore et al rightly
suggest that nonionizing electromagnetic fields are characterized
by a number of parameters. In fact, because fields are uniquely
characterized only when the value of each parameter (frequency,
magnitude, orientation, duration, and so on) is defined, it is
generally not realistic to draw direct analogies between field
exposure and single chemical exposures. For example, "high
exposure" to fields refers only to the measured field magnitude,
and not necessarily to its biologic effectiveness.

Although epidemiologic studies have, in some cases, shown a correlation
between nonionizing electromagnetic field exposure and the development
of certain cancers, the studies have not yet clearly identified
specific details of nonionizing electromagnetic fields that might
be causative. Laboratory studies, conducted under well-defined
conditions, may be useful for determining whether the exposure
categories used in epidemiologic studies are adequate, or how
they can be improved.

Essentially, all studies of nonionizing electromagnetic fields
focus on magnetic field effects because magnetic fields are rarely
shielded (attenuated) in the inhabited environment (home, work,
or transportation). Furthermore, laboratory studies have shown
that magnetic fields can have direct action on biologic processes
(eg, Blackman et al [1]), and epidemiologic data appear to be
more strongly correlated with estimates of magnetic fields than
with electric field estimates [2].

The Need to Define "Dose"

Until now, most laboratory animal studies using particular cancer
models with various nonionizing electromagnetic field exposures
showed negative results. However, at least two models (a skin
tumor model [3,4], and a breast tumor model [5-7]have recently
shown positive effects under specific, precise nonionizing electromagnetic
field treatment conditions. These studies suggest the need for
refining what is meant by "dose" under nonionizing electromagnetic
field exposures.

Three particularly promising areas of laboratory research for
providing critical focus to research are:

1. Investigations of subtle distinctions in field exposure conditions
that are necessary to elicit biologic changes,

2. Determination of whether and how certain magnetic field exposures
may augment the action of chemicals known to influence cancer
development, and

3. Studies examining how certain magnetic field exposures may
induce changes in the production, availability, and action of
critical oncostatic hormones, such as melatonin.

Characterizing Field Exposure Conditions

Recent laboratory research suggests that nonionizing electromagnetic
field interactions with biologic systems may be resonance-based
to some extent. Several resonance interaction models, with different
degrees of predictivity and validation, exist. For example, the
ion cyclotron resonance (ICR) model [8] identifies combinations
of alternating-current (AC) frequency and direct-current (DC)
magnetic flux density required to establish resonance conditions
for biologically active ions. Lednev [9] augmented the ICR model
by including the predicted influence of the AC magnetic field
flux density (Bac) on the resonance process.

More recently, the ion parametric resonance (IPR) model [10] identified
a Bac-based response form different from the Lednev model, and
considers the combined influence of multiple ion resonances at
any given exposure condition. This IPR model has growing experimental
support [11,12]. Other well-established magnetic resonance models,
such as nuclear magnetic resonance and electron spin resonance,
also appear to be useful in identifying the precise conditions
of nonionizing electromagnetic field exposure to create biologic
changes [13]. In each model, fundamental magnetic field conditions
must be closely controlled, including AC frequency(ies), AC and
DC flux densities, and the relative orientation between the AC
and DC magnetic field vectors.

Nonionizing electromagnetic fields may also interact through induced
electric current in conductive biologic materials. It has been
suggested, for example, that externally generated electric transients
may lead to more biologically significant induced currents within
the body.

Other studies examined the time signature of exposure conditions.
Historically, epidemiologic studies assumed that time-weighted
average (TWA) magnetic flux density was a sufficient measure of
exposure. The most compelling of these studies [14] showed that
averaging exposure intensity over 1-year periods produced a higher
correlation with disease incidence than spot measurements taken
for only a 15-minute-period. However, several recent studies (for
example, Morgan and Nair [15]), challenge the use of the TWA metric,
and have searched for more appropriate measures.

Synergy With Chemicals

The biologic effects of nonionizing electromagnetic field exposures
may be influenced by the presence of certain chemicals within
the body. These include environmental chemicals, specifically,
tumor promoters, and indigenous signaling molecules, such as hormones.
Tumor promoters are believed to influence biologic systems only
when their concentrations are above a certain threshold value.
As noted by Salvatore et al, it is possible that magnetic fields
can change (reduce) that apparent threshold.

Hormones normally carry information between tissue systems to
provide timely stimuli, causing specific tissue/cell metabolic
responses. For example, the pineal hormone, melatonin, dramatically
influences the body's circadian clock. Melatonin is the most potent
free-radical scavenger in the body (with potential implications
for protection against cancer-initiating events) [16] and also
acts as a modulator of gap junction intercellular communication
[17], an information channel that both coordinates cellular functions
and allows neighboring cells to control the growth of "initiated"
cells. Recent studies suggest that nonionizing electromagnetic
field exposures may control the amount of circulating melatonin
and influence its specific functions, making the biologic system
more susceptible to disease.

Summary

Because nonionizing electromagnetic fields cannot be characterized
solely by their magnitude, they are more complicated than earlier
studies have assumed, and hence, the single chemical analogy is
inappropriate. Present research suggests that (1) specific combinations
of nonionizing electromagnetic fields may be more effective at
creating biologic effects than others, and (2) synergistic effects
between chemicals and nonionizing electromagnetic fields are possible
within a biologic system. It is too early to claim categorically
that nonionizing electromagnetic fields do or do not have carcinogenic
properties. More directed research is clearly required to elucidate
the biologic significance of ubiquitous nonionizing electromagnetic
fields exposure(s).